Combinators for `ExtX` values

[andy-morris]

In Juvix we have an extension HRAnn which is literally just the product of HR and Ann. So it would be nice if we could express that with something like

module HR where
data T
hrExtTerm :: ExtTerm
hrExtTerm = ...
extendTerm "Term" [t|T|] hrExtTerm

module Ann where
data T
annExtTerm :: ExtTerm
annExtTerm = ...
extendTerm "Term" [t|T|] annExtTerm

module HRAnn where
import HR; import Ann
data T
extendTerm "Term" [t|T|] (hrExtTerm `pair` annExtTerm)